What Is an Ion?
An ion is simply an atom or molecule that is electrically charged. Ionization is the process of electrically charging an atom or molecule by adding or removing electrons. Ions can be positive (when they lose one or more electrons) or negative (when they gain one or more electrons). A gas is considered ionized when some or all the atoms or molecules contained in it are converted into ions. Plasma is an electrically neutral gas in which all positive and negative charges—from neutral atoms, negatively charged electrons, and positively charged ions—add up to zero. Plasma exists everywhere in nature; it is designated as the fourth state of matter (the others are solid, liquid, and gas). It has some of the properties of a gas but is affected by electric and magnetic fields and is a good conductor of electricity. Plasma is the building block for all types of electric propulsion, where electric and/or magnetic fields are used to push on the electrically charged ions and electrons to provide thrust. Examples of plasmas seen every day are lightning and fluorescent light bulbs.

The conventional method for ionizing the propellant atoms in an ion thruster is called electron bombardment. The majority of NASA's research consists of electron bombardment ion thrusters. When a high-energy electron (negative charge) collides with a propellant atom (neutral charge), a second electron is released, yielding two negative electrons and one positive ion. The ionization process in a xenon ion thruster is shown here:

 e^- + Xe⁰ => Xe⁺ + 2e^-

An alternative method of ionization called electron cyclotron resonance (ECR) is also being researched at NASA. This method uses high-frequency radiation (usually microwaves), coupled with a high magnetic field to heat the electrons in the propellant atoms, causing them to break free of the propellant atoms, creating plasma. Ions can then be extracted from this plasma.

Ion Thruster Operation
Modern ion thrusters use inert gases for propellant. The majority of thrusters use xenon, which is chemically inert, colorless, odorless, and tasteless. The propellant is injected from the downstream end of the thruster and flows toward the upstream end. This injection method is preferred because it increases the time that the propellant remains in the chamber.

In a conventional ion thruster, electrons are generated by a hollow cathode, called the discharge cathode, located at the center of the engine on the upstream end. The electrons flow out of the discharge cathode and are attracted to the discharge chamber walls, which are charged to a high positive potential by the thruster’s power supply.

The electrons from the discharge cathode ionize the propellant by means of electron bombardment. High-strength magnets are placed along the discharge chamber walls so that as electrons approach the walls, they are redirected into the discharge chamber by the magnetic fields. By maximizing the length of time that electrons and propellant atoms remain in the discharge chamber, the chance of ionization is maximized, which makes the ionization process as efficient as possible.

In an ion thruster, ions are accelerated by electrostatic forces. The electric fields used for acceleration are generated by electrodes positioned at the downstream end of the thruster. Each set of electrodes, called ion optics or grids, contains thousands of coaxial apertures. Each set of apertures acts as a lens that electrically focuses ions through the optics. NASA’s ion thrusters use a two-electrode system, where the upstream electrode (called the screen grid) is charged highly positive, and the downstream electrode (called the accelerator grid) is charged highly negative. Since the ions are generated in a region of high positive and the accelerator grid’s potential is negative, the ions are attracted toward the accelerator grid and are focused out of the discharge chamber through the apertures, creating thousands of ion jets. The stream of all the ion jets together is called the ion beam. The thrust force is the force that exists between the upstream ions and the accelerator grid. The exhaust velocity of the ions in the beam is based on the voltage applied to the optics. While a chemical rocket’s top speed is limited by the thermal capability of the rocket nozzle, the ion thruster’s top speed is limited by the voltage that is applied to the ion optics (which is theoretically unlimited).

Because the ion thruster expels a large amount of positive ions, an equal amount of negative charge must be expelled to keep the total charge of the exhaust beam neutral. A second hollow cathode called the neutralizer is located on the downstream perimeter of the thruster and expels the needed electrons.

The Electric Propulsion System
The ion propulsion system (IPS) consists of five main parts: the power source, power processing unit (PPU), propellant management system (PMS), the control computer, and the ion thruster. The IPS power source can be any source of electrical power, but solar and nuclear are the primary options. A solar electric propulsion system (SEP) uses sunlight and solar cells for power generation. A nuclear electric propulsion system (NEP) uses a nuclear heat source coupled to an electric generator. The PPU converts the electrical power generated by the power source into the power required for each component of the ion thruster. It generates the voltages required by the ion optics and discharge chamber and the high currents required for the hollow cathodes. The PMS controls the propellant flow from the propellant tank to the thruster and hollow cathodes. Modern PMS units have evolved to a level of sophisticated design that no longer requires moving parts. The control computer controls and monitors system performance. The ion thruster then processes the propellant and power to perform work. Modern ion thrusters are capable of propelling a spacecraft up to 90,000 meters per second (about 200,000 miles per hour (mph)). To put that into perspective, the space shuttle is capable of a top speed of around 18,000 mph. The tradeoff for this high top speed is low thrust (or low acceleration). Thrust is the force that the thruster applies to the spacecraft.

Modern ion thrusters can deliver up to 0.5 Newtons (0.1 pounds) of thrust, which is equivalent to the force you would feel by holding nine U.S. quarters in your hand. To compensate for low thrust, the ion thruster must be operated for a long time for the spacecraft to reach its top speed. Ion thrusters use inert gas for propellant, eliminating the risk of explosions associated with chemical propulsion. The usual propellant is xenon, but other gases such as krypton and argon may be used.

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